1. Field of the Present Invention
This invention relates to the field of magnetic media storage systems.
2. Background Art
Mass storage for computer or other information systems is typically provided by magnetic media storage systems such as rigid or flexible disk storage systems. A rotating disk having a magnetic media layer on the surface is accessed by a "read/write" head which is used to store and retrieve information from the disk surface. To store information on a magnetic media disk, flux reversals are induced in the magnetic particles comprising the surface. When a magnetic read/write head is passed over the flux reversals, a signal is induced in the head which can be decoded to provide information.
Typically, data is stored on a magnetic disk in a series concentric "tracks" on the surface of the disk. The read/write head moves back and forth radially on the disk so that it can be selectively positioned over one of the tracks. Once in position over a track, the head remains in place as the track rotates beneath it allowing the head to read or write data on the track.
To effectively read and write data, it is necessary that the position of the tracks with respect to the head be known. In addition to knowing which track a head is over, it is necessary to know where on that particular track the head is positioned.
In the prior art, position information is provided through the use of servo patterns. A servo pattern is a permanent pattern on a storage disk which can be used to provide position information. The servo pattern is detected by the head and, by properly decoding the servo pattern, signifies track position.
Generally, two types or servo schemes are implemented in the prior art, sector servo and dedicated surface servo.
In the dedicated surface servo scheme, one side of a data disk contains only servo information. A servo head is positioned over the surface. One or more other read/write heads are positioned over other disk surfaces used for data storage and retrieval. These read/write heads are in a fixed position relative to the servo head and are coupled mechanically thereto. Therefore, if the radial position of the servo head is known, the position of the read/write heads is known as well. The servo pattern is typically a series of concentric tracks corresponding to the number of data tracks on a disk. Each track is encoded with a sequence of magnetic transitions disposed in a plurality of servo frames. Each frame typically contains a sequence of magnetic transmissions which encode both an analog measure of servo head radial position, as well as single bit of digital information used to encode index information.
In addition to knowing the track over which a read/write head is positioned, it is necessary to know at what circumferential position in the track the head is located. Generally, an "index" is defined at a specific circumferential location in the servo pattern to indicate the start/end location of each data track.
The index on each track consists of a unique sequence of servo frames which jointly encode a specific binary sequence of bits which is defined as the index position. In the prior art, a five to eight bit sequence is generally utilized to define the index. Decode circuitry coupled to the servo continually searches for the specific index bit sequence to determine index position. When the complete index sequence is detected, a track index point signal is emitted by the index detection circuits for use in other elements of the system of which the disk drive is a part.
If for some reason index is lost, the decode circuitry reads the entire servo track until the index sequence is found again. A disadvantage of this prior system is that it is not fault tolerant. If a servo frame in the index area is damaged or servo information is lost, the bit sequence will never be detected and an index will not be established. In other instances, if the servo information itself is damaged, a false index may be indicated. A loss of index can result in a data track being unavailable for use, loss of data, or data errors.
In the sector servo method, bursts of servo information are disposed on a disk surface in between data areas. The same head is used to reproduce both data and servo information. As in the dedicated servo approach, each servo burst contains both analog track position information, and digital information. The digital portion of the servo burst can include both a track address field, and a single index bit.
Betts et al in U.S. Pat. No. 4,511,938 shows a typical implementation of sector servo and provides for a single bit in the servo burst to encode index point. He does not disclose means for decoding this bit in context in a fault-tolerant manner to provide a reliable index indication despite possible flaws in one disk surface which could render one or more servo bursts unreadable.
Use in the prior art of binary sequences having a high degree of selfcorrelation is known, however these codes have been applied primarily either in detection schemes in radar or in data transmission and not in the servo systems of disk files. Barker, "Group Synchronizing of Digital Systems" Signals Research and Development Establishment; reprinted in "Communication Theory", W. Jackson New York Acadamic Press, cites a simple algorithm for determining highly selfcorrelated sequences by juxtiposition of simpler sequences. The Barker article does not treat more general sequences which are used in the present invention, and which are obtainable by computer search techniques nor does it propose elaboration of the code sequence to minimize misdetection in the presence of other admissible data sequences.
Therefore, it is an object of the present invention to provide a servo index pattern detection scheme which is fault tolerant.
It is yet another object to provide a servo pattern detection scheme which is fault tolerant for error bursts in which three or fewer consecutive dedicated servo frames or sector servo bursts are damaged.
It is still another object of the present invention to provide a servo index detection scheme which additionally provides means for performing physical track addressing of individual tracks on the servo surface.